Abstract: Desorption and dissolution often control the mobility and availability of phosphorus (P) in the natural environment. In this study, P desorption was compared from a soil receiving either long-term inorganic or biosolid fertilization as a part of a long-term field scale research project. A continuous-flow desorption method was used to measure cumulative P desorption over time, and P K-edge X-ray absorption near edge structure spectroscopy was used to determine the chemical species removed from the soil samples by desorption. The cumulative amount of P released in the inorganic fertilizer–amended soil was higher (895 vs. 573 mg kg−1), and the rate of P release was much faster (k = 0.012 vs. 0.005 m−1) than that of the biosolids-amended soil. The kinetics data were best described by the parabolic diffusion equation (r2 = 0.98–0.99), suggesting that P desorption was mass-transfer limited or that intraparticle diffusion could be the rate-limiting step. The X-ray absorption near edge structure results indicated that dissolution of calcium and iron phosphate minerals occurs in addition to desorption of P from the exchangeable sites. These observations suggest that the redistribution between aqueous, adsorbed, and precipitated phosphate (PO43−) species occurs rapidly when solution P concentrations are depleted.